Re: [PATCH v4 08/11] mtd: brcmnand: add BCM63138 support

Brian Norris
2015-05-13T20:24:28Z

On Wed, May 13, 2015 at 10:02:49PM +0200, Arnd Bergmann wrote:
> On Wednesday 13 May 2015 12:45:21 Brian Norris wrote:
> > I could still avoid one pointer chase and one extra memory allocation by
> > embedding 'struct brcmnand_soc' in a 'struct bcm63138_nand_soc'. e.g.:
> >
> > struct bcm63138_nand_soc {
> > void __iomem *base;
> > struct brcmnand_soc soc;
> > };
> >
> > static bool bcm63138_nand_intc_ack(struct brcmnand_soc *soc)
> > {
> > struct bcm63138_nand_soc *priv;
> > priv = container_of(soc, struct bcm63138_nand_soc, soc);
> >
> > ...
> > }
> >
> > static int bcm63138_nand_probe(...)
> > {
> > struct bcm63138_nand_soc *priv;
> >
> > priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
> > ...
> > return brcmnand_probe(pdev, &priv->soc);
> > }
>
> That would make struct brcmnand_soc an empty structure, right?

No, it still contains the function pointers for our callbacks, which is
the entire point. I guess it's more of a 'nand_soc_ops' structure than a
'nand_soc' pointer now though.

> I think that's fine though, at least it avoids passing void pointers
> and it avoids one of the two allocations you do.
>
> There is another variation of this model, which some drivers use:
>
> static int bcm63138_nand_probe(...)
> {
> struct bcm63138_nand_soc *priv;
> struct brcmnand_controller *controller;
>
> controller = brcmnand_controller_alloc(dev, sizeof (*priv));
>
> priv = brcmnand_controller_priv(controller);
>
> ...
>
> return brcmnand_register(controller);
> }
>
> struct brcmnand_controller *brcmnand_controller_alloc(struct device *pdev, size_t extra)
> {
> struct brcmnand_controller *p = dev_kzalloc(dev, sizeof(*p) + extra);
>
> ...
>
> return p;
> }
>
> void *brcmnand_controller_priv(brcmnand_controller *p)
> {
> /* extra data follows at the next byte after the controller structure */
> return p + 1;
> }

Ah, so this allows the driver to still be agnostic about the contents of
brcmnand_controller.

> Some subsystem maintainers prefer this model over the other one, up to you.

I'll probably stick with mine. But thanks for the suggestion. I'll keep
it in mind. I was actually thinking of imitating this model for other
larger portions of drivers/mtd/nand/, to aid in bounding what drivers
are expected to do vs. allowing the core subsystem to handle things.

Brian